建筑设计英文文献及翻译 第3页
alkaline conditions,pH.13),and are able to react with and modify the bentonite barrier(Cuevas et al.,2002;Ramirez et al.,2002;Savage et al.,2002).These early cement pore waters consist of K+(0.2–0.5 M),Na+(0.05–0.2 M)and OH-(0.3–0.7 M),that will be transported by diffusion and possibly by advection due to bentonite suction if this material is emplaced under unsaturated conditions.Two main stages have been distinguished during concrete alteration due to leaching and interaction with external porewater.
A first hyper alkaline early and relatively short-term stage is characterized by the leaching of dissolved alkali hydroxides that will control the initial pHs in the range of 13–14.During this stage,the alteration of bentonite is characterized by montmorillonite dissolution.Sodium and potassium zeolites(e.g.,analcime,phillipsite)are formed,as a function of the Na+or K+ availability from the concrete(Bauer&Velde,1999;Vigil de la Villa et al.,2001;Ramirez et al.,2005).
In a second alkaline long-term stage,the system is buffered by the portlandite(Ca(OH)2)dissolution at pH~12.5(measured at 25℃)(Read et al.,2001;Savage et al.,2002;Gaucher et al.,2004).Ca2+s the predominant cationic species in the leaching solution of the cement mortar.In this case,zeolites are replaced presumably by cement phases such as calcium silicate hydrates(C–S–H)in the altered bentonite region.
Other authors(Berner,1992;Taylor,1997)include a third stage when the buffer capacity of portlandite is exhausted,and pH decreases to values below~12.5.During this stage the incongruent dissolution of C–S–H gels and other cement phases determine the pH buffering capacity from the concrete side.
Reactive transport modelling studies in this specific field have been carried out predicting the alkaline plume effects on the bentonite and claystone barriers for the next hundreds to thousands of years(De Windt et al.,2001;Savage et al.,2002;De Windt et al.,2004;Gaucher et al.,2004;Ma¨der&Traber,2004;Soler&Ma¨der,2005;Traber&Ma¨der,2008).There is,however, still a lack of well characterized laboratory studies to test, modify and enlarge present thermodynamic and kinetic databases that validate the predicted alteration reactions –both,within the mortar and the clay-based barrier.
In this work,the modelling simulations were carried out by using the geochemical code CrunchFlow(Steefel,2006),firstly,to simulate laboratory transport cell experiments where an alkaline solution(NaOH 0.25 M)was forced to pass though a cement mortar disk and a compacted bentonite column.Experimental results(Ferna′ndezet al.,2006)provided the identity of secondary minerals to work with in this modelling study.Secondly,once the kinetic conditions were evaluated,long-term modelling of the host rock–concrete–bentonite system was performed for 105
years.
2.Summary of experimental results
Percolation cell experiments were performed with a hyper alkaline NaOH 0.25 M(pH 13.4)solution,which mimics the early stage of pore water of the leached cement.The solution was injected into the cement mortar–compacted bentonite system.A cylindrical column of 0.7 cm height and 5.5 cm diameter of CEM-I Ordinary Portland Cement (OPC)was used.This cement provides a Ca(OH)2 dominated system which will be able to dissolve if concrete–bentonite reaction causes a significant pH decrease.The bentonite column was uniaxially compacted to 1.4 g .cm-3(dry density)into the dimensions of 2.2 cm height and 7.0 cm diameter.The effluent composition was tested once a month during the experimental time up to one year.One-month and six-month experiments were also carried out to characterize intermediate stages(see Fernandez et al.,2006,for details).The experiments were performed at 25 and 120℃,to obtain the extent of transformations also as a function of temperature.
At high temperature(120℃),three stages can be differentiated in the geochemical evolution of the system,based on changes in effluent composition.The first stage (1–2 months)is characterized by the leaching of soluble salts initially contained in the bentonite.Chloride measured in the effluent decreases sharply from its initial concentration during the first weeks of reaction.In the case of sulfates,the analytical measurements taken in the effluent samples decrease gradually due to the dissolution of sulfate phases in the cement mortar(ettringite and any residual gypsum/anhydrite).The second stage(2–6 months)is dominated by dissolution/precipitation of sec-
ondary minerals associated with a gradual pH increase from 8 to 12 due to the C–S–H(Ca-Si-hydrate)–montmorillonite reaction(Cuevas et al.,2006).As a result of this second stage,montmorillonite dissolves creating preferential pathways for fluid migration.At the same time,some clay aggregates become coated with cement hydration phases at the cement mortar–bentonite interface.These coatings blocks the porosity at the meso-scale(< 500A) ,producing a drastic reduction of the external specific surface area.In a third stage(6–12 months),once clay aggregates have been sealed,the alkaline solution is able to flow through these pathways,scarcely interacting with the initial clay,associated with a pH increase to 12.5 and an increase in the hydraulic conductivity to twice the value measured at 120 ℃ initially(5.57±1.53×10-13m.s-1)毕业论文
http://www.youerw.comThe main mineralogical transformations obtained at high temperature are the formation of secondary Mgclays,the formation of zeolites(analcime could be quantified by XRD and observed by SEM)and the precipitation of low Ca/Si ratio crystalline C–S–H of tobermorite-type in the first millimetres from the interface.The degree of crystallization of C–S–H decreased with the distance from the interface.
At low temperature(25℃),the hydraulic conductivity decreases with time.This also reduces the thickness of the alteration zone.At 25℃,precipitation of C–S–H gels and brucite in the cement mortar–clay contact plugged the pore space preventing the solution to get into the bentonite. No mineralogical quantification of new minerals could be identified by XRD,but the same 无耻悲鄙下流的网.学,网总是抄优|文,论-文.网 detected by SEM as observed at 120℃.C–S–H gels are not crystalline and show different morphologies,from fibrous to Ca-rich smooth coatings.The C–S–H morphology is a function of the temperature applied in the experiments.
The detailed description of these experiments can be found in Ferna′ndez et al.,2006.Mineralogical transformations observed in bentonite as a function of temperature,pH and altered thicknesses are shown in Table 1.
3.Modelling tools and basic assumptions
The geochemical reactive transport code CrunchFlow (Steefel,2006),the updated version of the GIMRT/OS3D code(Steefel&Yabusaki,1996;Steefel,2001) was used to simulate the experiments carried out at 25℃ and 120℃.The code incorporates a kinetic treatment for mineral dissolution/precipitation.
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